This invention relates generally to two-wire electronic transmitters adapted to convert an analog voltage representing a metered value into a corresponding current which is conveyed over a pair of wires to a receiving station, and more particularly to a transmitter of this type which incorporates a counter to totalize the metered value without disturbing the operation of the transmitter.
Two-wire transmitters are useful in process control systems for converting a sensed value into a current signal which is transmitted to a receiving station for operating indicators, recorders, and process control systems. One important advantage of a two-wire transmitter of this type is that the same wires serve not only to convey the current signal from the transmitter to the station but also to conduct a direct operating voltage from a power supply to the station to the transmitter.
The dual use of the power supply leads as the signal output eliminates the need for extra wires in remote signal applications. Also, a current output minimizes susceptibility to voltage noise spikes and eliminates line drop problems. A typical, commercially-available, two-wire transmitter making use of linear integrated circuits is the model LH 0045/LH 0045 C two-wire transmitter manufactured by National Semiconductor Corporation and described in their instruction bulletin covering this transmitter.
In many practical applications for a two-wire transmitter, the metered value to be conveyed is the output of a measuring instrument which is expressed in terms of a signal of varying frequency. For example, among the flowmeters which yield an output voltage whose frequency is proportional to flow rate are positive displacement meters, such as those operating on the turbine principle. Also, in a swirl type or vortex-shedding flowmeter in which fluidic oscillations are produced in a flow tube, these oscillations are converted by a transducer into an electrical signal whose frequency depends on flow rate.
In order to transmit a varying-frequency signal, the signal must first be changed into an analog voltage whose magnitude varies as a function of frequency, this voltage then being converted in the transmitter into a corresponding current, usually in the range of 4 to 20 mAdc, and in some instances in the range of 10 to 50 mAdc.
It is important in some industrial applications for billing or other purposes to determine the total flow that has passed through the meter. While a total flow reading may be obtained by means of a totalizing counter associated with the meter, it has not heretofore been possible to incorporate a totalizing counter in a two-wire transmitter without disturbing the normal operation thereof.